With the invention of polyacrylate for horticultural end uses, it has become possible to retain soil moisture for longer periods of time, especially especially during long periods of moisture deficiency. As the soil dries out, the zone surrounding the super absorbent particulate retains its moisture, creating a more desirable environment for the plant roots in these zones with a consistent supply of moisture.
Water availability is the key factor limiting plant growth and development especially in dry and semi-arid ecosystems. Super absorbent polymers help soil keep water after raining or irrigation, and release water gradually during plant growth.
A number of water absorbing substances have been disclosed as additives for soil to provide a consistent moisture source for plants. An important criterion for successful usage is the ability of the water absorbing substance to maintain its water retention characteristics over a series of wet/dry cycles.
Sodium polyacrylate is the dominant super absorbent polymer in use today with over 90% market share. As it comes at a cheaper price, profiteers often market sodium polyacrylate for agriculture uses. However, its major application is as a body fluid absorbent in hygienic disposal products, particularly in diapers and sanitary napkins where rapid water absorption is an essential requirement, with a strong affinity for water.
During dry periods, sodium polyacrylate tends to condense and form crosslinks that inhibit re-swelling when it is re-wetted. Even when used in situations where a limited number of wet/dry cycles are experienced, sodium polyacrylate inhibits plant growth or in some cases is even toxic to plants. This inhibition of plant growth or toxicity is believed to arise because the sodium ions in the sodium polyacrylate network are easily exchangeable and are adsorbed by clay particles or tend to undergo exchange with cations on the surface of plant roots creating a condition which is analogous to an alkaline soil, which generally tends to adversely affect or inhibit plant growth. Sodium polyacrylate gel offers potential for use in horticultural end uses, provided that the tendencies to inhibit plant growth or toxicity towards plants can be reduced.